Ozone in photochemical smog

Materials The damage that air pollutants can do to some materials is well known ozone in photochemical smog cracks rubber, weakens fabrics, and fades dyes hydrogen sulfide tarnishes silver smoke dirties laundry acid aerosols ruin nylon hose. Among the most important effects are discoloration, corrosion, the soiling of goods, and impairment of visibility. 

The photodissociation of N02 is the only definitely established process for the formation of ozone in the troposphere and must be held responsible also for the generation of ozone in photochemical smog. The reaction of alkylperoxy radicals with oxygen, ROO + 02— R0 + 03 has occasionally been invoked—for example, by Cadle and Allen (1970)—but according to... 

Here, R, represents an alkyl group containing one carbon atom less than R, so that R,0- = RiCH2CK The net reaction converts one molecule of n-alkane into one molecule of ketone and aldehyde each, and it oxidizes three molecules of NO to N02. The subsequent photodissociation of N02 is the source of ozone in photochemical smog. Note that the OH radical initiating the reaction sequence is regenerated so that it can continue the chain reaction. 

Thus, a large buildup of ozone in photochemical smog requires a plentiful source of NO2. At one time, this source was believed to be reaction (20.31). 

The atmospheric chemistry of nitrogen is quite complex and involves literally hundreds or thousands of chemical reactions. Although the fluxes are much smaller than the biological fluxes, these processes are important for a variety of reasons, including impacts on climate, stratospheric ozone, and photochemical smog. In this section we present an overview of the most important processes. 

These reactions are important in a cycle that oxidizes CO and hydrocarbons and produces ozone, in the presence of NO (NO + N02). In photochemical smog, ozone can build up to... 

Nitric acid synthesis, platinum-group metal catalysts in, 19 621 Nitric acid wet spinning process, 11 189 Nitric oxide (NO), 13 791-792. See also Nitrogen oxides (NOJ affinity for ruthenium, 19 638—639 air pollutant, 1 789, 796 cardioprotection role, 5 188 catalyst poison, 5 257t chemistry of, 13 443—444 control of, 26 691—692 effect on ozone depletion, 17 785 mechanism of action in muscle cells, 5 109, 112-113 oxidation of, 17 181 in photochemical smog, 1 789, 790 reduction with catalytic aerogels, l 763t, 764... 

Ozone, O3, is an essential molecule in the ozone layer. It filters out some of the UV radiation that would otherwise destroy life on Earth. At ground level, ozone is a component in photochemical smog. Draw the Lewis structure for ozone, and indicate whether or not ozone is polar. Can ozone exist in resonance forms ... 

Eye irritation and lacrimation are by far the most widespread and common symptoms clearly associated with increased photochemical-oxidant pollution. Although photochemical oxidant is customarily measured as ozone, ozone itself is not a primary eye irritant. Thus, the eye irritation associated with increased photochemical oxidants is probably due to some or many of the complex organic oxidants produced in photochemical smog. Richardson and Middleton,noted nearly 20 yr ago that eye irritation could be expected in a considerable portion of the population when the oxidant concentration reached 200 ng/m. ... 

The development of models requires more knowledge about the chemical, physical, morphologic, and flow properties of the mucus layer the kinetics of the reactions of ozone in the mucus and tissue layers and the molecular diffusivity of ozone in these layers. Similar information is needed for the hydroperozy and singlet oxygen, O, (a A), free radicals, which are reactive interme ates in photochemical smog. 

In recent years there has been increased interest in elucidating the biochemical and physiological responses of plants to air pollutants. Ozone has received particular scrutiny because of its importance in photochemical smog. The effects of ozone on the spectrum of physiological and biochemical systems has been analyzed in plant systems both vivo and vitro to... 

Fig. VI-9. Concentration-lime history or reactants and some products in the photooxidation of C3H. Mixtures of C,H4 + NO + NOj in air were irradiated by simulated sunlight in smog chamber. Reprinted with permission from H. Niki, C. E. Daby, and B. Weinstock, in Photochemical Smog and Ozone Reactions, R. F. Gould, Ed., American Chemical Society, Washington, D. C. Copyright by the American Chemical Society, 1972. 

Nitrogen Oxides. Nitrogen dioxide (NO2), a gas found in photochemical smog, is also a pulmonary irritant and is known to lead to pulmonary edema and hemorrhage. The main issue of concern is its contribution to the formation of photochemical smog and ozone, although nitrogen oxides also contribute to acid deposition. 

H. Niki, E. E. Daby, and B. Weinstock, in Photochemical Smog and Ozone Reactions, R. F. Gould, Ed., American Chemical Society, Washington, D.C., 1972,... 

One of the first atmospheric chemical cycles to be well documented involves the production of high levels of ozone and photochemical smog. This cycle is ubiquitous in polluted air in any urban area, but was first studied in detail in Los Angeles, California, where the problem is particularly bad due to the high vehicle density and low ventilation rate of the Los Angeles Basin. The set of all possible atmospheric reactions that occur is complex, so only the basic 03-N0x cycle and one representative hydrocarbon pathway are presented here. 

In photochemical smog episodes, secondary air pollutants such as ozone, nitrogen dioxide, aldehydes, and peroxyacetyl nitrate are formed as a result of the chemical interaction of the primary air pollutants, principally nitric oxide and hydrocarbon vapors, with sunlight and air (Fig. 2.6) [49]. This interpretation of the processes involved has been verified by smog chamber experiments (Fig. 2.7), and has since been confirmed by field measurements as the sensitivity of ambient air instrumentation has improved [50, 51]. In photochemical smog episodes, it is the secondary pollutants that cause severe eye irritation and upper respiratory effects felt by people and at the same time causes serious damage to plants. 

These reactions are important in a cycle that oxidizes CO and hydrocarbons and produces ozone, in the presence of sufficient NO. In photochemical smog, ozone can build up to unhealthy levels of several hundred parts per billion (ppb) as a result of these reactions. There are many other reactions that occur, some of which may be significant at various times, including the destruction of O3 by NO, production and loss of HONO (nitrous oxide) and peroxyacetyl nitrate (PAN), and further oxidation of CH2O. These reactions, and many more, represent a complex set of chemical interactions. For our purposes here, it is only necessary to note the major... 

Olefins. Olefins are the most reactive class of hydrocarbons in photochemical smog and have been studied extensively (I, 17, 18, 19). In general, as was perhaps first noted by Schuck and Doyle (20), the mechanism for olefin decomposition apparently involves electrophilic attack (by atomic oxygen, ozone, and other species) on the double bond. Thus, for most of the chemical reactions related to smog formation, olefin reactivity generally increases with additional alkyl (or other electron-donating) groups attached to the two carbon atoms joined by the double bond. 

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